How to make lego unbreakableI think, as @zovits mentioned in their answer, you should clarify what kind of model you are building and what kind of damage you expect it to withstand. Also keep in mind that some pieces are more fragile than others. Finally, consider that the Lego company just glues models when they want them permanently assembled.

Large trap door - alternative to door with frame 4595708@Ana You're welcome. There are, of course, infinitely many other ways to do it. But this is using one of the newer, available-in-stores techniques. And using different pins to hold the plates together allows you to have looser or stiffer trap doors.

Advanced set to 5.5 years oldThe car models I suggested are of the same difficulty add the ones you asked about. They should also be comparable prices. The two Creator expert fairground rides are expensive but definitely not easy. However, the older sets are usually much more expensive than the later sets. If you can find them at a good price, they'll do just fine. But there is little technical difference between the creator car models on a yearly basis. They just refresh the model with new colors or updated parts.